Numerous methods for measuring soiling of screens e.g. in motor vehicles are known. In DE 10230200 a detector is described to identify objects on a surface of a transparent element. In this case, the detector is at least approximately focused on the surface and for identifying soiling the contrast distribution of the recorded frames is evaluated. A further method to identify objects located on a windscreen is shown in DE 19749331. For this purpose, a sensor array is focused on the exterior of the windscreen. An analysis of the spatial frequencies in the recorded frame provides information on a soiling of the windscreen. Due to the focusing of the camera on the surface both arrangements cannot be used for other camera-based applications, e.g. for coverage of the surroundings. In DE 19700665 a camera-based sensor is described, which detects foreign materials on a transparent screen. For this purpose the image of a firmly installed light emitting reference object is recorded through the screen and is compared with a reference image. It is disadvantageous with this arrangement that a reference object is needed to measure soiling.